replace dictionary value at key using linq

Question

Edit: I don't feel this was duplicate- I did fail to elaborate clearly that I wanted to know an efficient way to pad char's to the left of my dictionary values. The question I marked as answer should clarify as it is a very different answer than the answer in other thread. Sorry, I worded it wrong.

I am trying to verify that all the values within my Dictionary are a certain length, and if they are not, pad them with '0' to the left. I know "what" the issue is, I am reading through the dictionary's KVP and then modifying it which of course throws an exception, but I am not sure how to modify my code to make it "legal"

class prefixMe
{
    Dictionary<string, string> d = new Dictionary<string, string>();

    public prefixMe()
    {
        d.Add("one", "123456");
        d.Add("two", "678");
        d.Add("three", "13");

        foreach(KeyValuePair<string,string> k in d)
        {
            StringBuilder sb = new StringBuilder(k.Value);
            while(sb.Length != 10)
            {
                Console.WriteLine("Key:{0}\tValue:{1}", k.Key, sb);
                sb.Insert(0, '0');
            }
            d[k.Key] = sb.ToString();
            //k.Value.Replace(k.Value, sb.ToString());// = sb.ToString();
            Console.WriteLine("Key:{0}\tNew Value:{1}", k.Key, d[k.Key]);
            break;
        }

    }
}

The break; stops the code from throwing an error, but also makes it so it does not go through for the other values as it breaks out of the foreach loop.

The result should be:

0000123456
0000000678
0000000013

You can avoid the issue with a helper function for adding to the list: `void add(string key, string val) {d.Add(key, val.PadLeft(10, '0'));}` — 001, May 23 '14 at 13:39

Maarten · Accepted Answer · 2014-05-23T13:44:32.427

7

Use

foreach(KeyValuePair<string,string> k in d.ToList())

instead of

foreach(KeyValuePair<string,string> k in d)

This creates a list of the key-value-pairs, through which you then iterate. This means that in the loop you can touch the dictionary.

If you don't mind creating a new dictionary, then you can also replace the dictionary quite easily using Linq.

d = d.ToDictionary(x => x.Key, x => x.Value.PadLeft(10, '0'));

edited May 23 '14 at 13:44

answered May 23 '14 at 13:39

Maarten

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upvoted. One liner Linq answer could replace 80% code in the question. – Hassan May 23 '14 at 13:53
1

This works perfect- no I am absolutely fine with making new dictionary- I am just new to LINQ. I figured there was a more efficient way to do this. Thank you! – Nyra May 23 '14 at 13:54
Also for what it is worth, I added this where clause to ignore null/empty values from being padded with all zero's (in my program I want them to remain null/empty) d = d.Where(x => !string.IsNullOrEmpty(x.Value)).ToDictionary(x => x.Key, x => x.Value.PadLeft(10, '0')); – Nyra May 23 '14 at 14:08

score 2 · Answer 2 · edited May 23 '17 at 10:33

2

Use a for loop instead of a foreach loop. This will allow you to iterate through the Dictionary and modify it in the same go.

EDIT: Upon further Googling/trying to compile a for loop through a dictionary, it seems you can't actually do this. I found How to modify key in a dictionary in C#, it seems you can't even change a key without removing it. So maybe this question will be helpful to you.

edited May 23 '17 at 10:33

Community

1
1

answered May 23 '14 at 13:36

eddie_cat

2,527
4
25
43

3

Could you add an example of how to iterate over a dictionary and changing it with a `for` loop? – sloth May 23 '14 at 13:42
1

As far as I know you can't iterate though a dictionary with a for loop. – toplel32 May 23 '14 at 13:42

Selman Genç · Answer 3 · 2014-05-23T13:43:28.973

1

Use PadLeft method, and do not break your loop on first iteration.

public prefixMe()
{
    d.Add("one", "123456");
    d.Add("two", "678");
    d.Add("three", "13");

    foreach(var k in d.ToList())
    {
        if(k.Value.Length != 10) 
          d[k.Key]  = k.Value.PadLeft(10, '0');
    }

}

ToList will allow you to modify your dictionary.

edited May 23 '14 at 13:43

answered May 23 '14 at 13:36

Selman Genç

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if this works, it would also be very helpful, I went with just transferring the entire dictionary and padding accordingly. Thank you though. – Nyra May 23 '14 at 14:00

score 1 · Answer 4 · answered May 23 '14 at 13:46

1

This should do the trick in a line

Dictionary<string, string> result = d.ToDictionary(p => p.Key,p=> p.Value.PadLeft(10, '0'));

answered May 23 '14 at 13:46

Mohamed

470
3
14

score -1 · Answer 5 · answered May 23 '14 at 13:36

-1

Use a for loop instead of foreach. In a foreach you cannot modify the collection (in your care d) because you are iterating on it

answered May 23 '14 at 13:36

Alexandru Chichinete

1,166
12
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2

Could you add an example of how to iterate over a dictionary and changing it with a `for` loop? – sloth May 23 '14 at 13:42
Check the bottom answer here: http://stackoverflow.com/questions/15241257/using-a-for-loop-to-iterate-through-a-dictionary – Alexandru Chichinete May 23 '14 at 13:45
That doesn't change the dictionary. – sloth May 23 '14 at 13:48
2

And makes looking up each item a O(n^2) operation. – Maarten May 23 '14 at 13:48

replace dictionary value at key using linq

5 Answers5